Various manufacturers have put on the market liquid crystal displaying apparatuses each using light emitting diodes (LED) as its backlight light source. The LED is a semiconductor element that emits light when a voltage is applied thereto in the forward direction thereof. When a positive voltage and a negative voltage are respectively applied to the anode and the cathode of the LED, a current flows in the LED at a voltage of several volts and the LED emits light. Control of turning on and off of, and control of the brightness of the backlight are enabled for each area by using the LEDs as the backlight light source and, therefore, the contrast can significantly be improved. The LED has a longer life and needs a smaller power consumption than those of a traditional fluorescent tube and, therefore, has an advantage of a higher energy-saving effect than that thereof.
When the LEDs are used as a backlight light source especially of a large-screen display, electric currents need to be supplied to many LEDs and, therefore, feedback control is executed to keep the current flowing in each of the LEDs constant (current feedback) using the LED power source as a switching power source (see, e.g., Patent Document 1).
FIG. 7 is a diagram of the configuration of a traditional LED lighting device and “100” therein denotes the LED lighting device. The LED lighting device 100 includes a converter circuit 2 including a switching power source 3, an LED driving circuit 4 to drive LEDs (1 to 5) of an LED circuit 5, and the LED circuit 5 including the plural LEDs (1 to 5). The LED driving circuit 4 is mounted as an integrated circuit (IC) chip including a known microcomputer and peripheral circuits. An output of a signal from the LED driving circuit 4 to an external circuit is controlled by the microcomputer.
The LED driving circuit 4 receives a dimming control signal from a main control portion (not depicted) to control a dimming duty ratio to a predetermined dimming duty ratio. The LED driving circuit 4 outputs a duty signal based on the dimming duty ratio from its feedback output terminal to the converter circuit 2, controls a start of the operation and discontinuation of the operation of the switching power source 3, and causes the switching power source 3 to generate a voltage V1 corresponding to the dimming duty ratio. The LED driving circuit 4: causes a switching element TR1 inserted between the LED circuit 5 and a ground line GND to be driven for switching based on the duty signal; causes the voltage V1 to continuously be applied to the LED circuit 5; and, thereby, causes the LEDs (1 to 5) to be turned on. The switching element TR1 is composed of, for example, a field effect transistor (FET).
The converter circuit 2 generates the voltage V1 to turn on the LEDs (1 to 5) included in the LED circuit 5 based, for example, on a reference voltage V generated from the power source voltage (+B) such as a battery. Though the converter circuit 2 is configured as a what-is-called step-down converter including a capacitor C, the converter circuit 2 may be of any one of a step-down, a step-up, and a step-up-and-down types.
The LED circuit 5 includes one or more LEDs. In the depicted example, the LED circuit 5 includes five LEDs of LEDs (1 to 5). An electric current i flowing in the LED circuit 5 is converted from a current to a voltage by a current detection resistor R1 and the voltage is input into a current detection terminal of the LED driving circuit 4. The LED driving circuit 4 outputs the value of the voltage input into the current detection terminal from its feedback output terminal to the converter circuit 2, and causes the converter circuit 2 to drive the LEDs (1 to 5) each with a constant current.
The image quality of recent liquid crystal displaying apparatuses is increasingly improved and a demand is present that the contrast ratio is increased as much as possible, to acquire clearer image quality. When LEDs are used as the backlight, the contrast ratio is determined based on the ratio of the maximal luminance and the minimal luminance of the LEDs. An increase of the contrast ratio is equivalent to an increase of the ratio of the maximal current: (the rated current) and the minimal current that flow in the LEDs.
The current flowing in the LEDs is determined by a current supplied by the LED driving circuit as described with reference to FIG. 7. The manufacturers of the liquid crystal displaying apparatuses each set target ratio of the maximal current to the minimal current to be caused to flow in the LEDs, to realize the desired contrast ratio. For example, the target ratio is set to be 2000:1. For example, when the maximal current supplied by the LED driving circuit is 250 mA, the target value of the minimal current is 0.125 mA.